Improving how excrement and carcass decay is handled

[jacobcook1995]

Description

This PR does three things:

1. stops excrement and carcass biomass from flowing into the litter model, as this meant that we were basically requiring it to decay twice (once in the animal model and once in the litter model) before it entered the soil.
2. adds tracking of excrement and carcass nitrogen and phosphorus. These links are really placeholders at the moment, which should be filled out once animal model stoichiometry is implemented.
3. adds a function to calculate the split between the "scavengable" and "decayed" pools. This was previously handled by a fixed constant split. The change I made basically does the same thing with an extra step, but (hopefully) makes the logic of what we are doing clearer

The excrement and carcass decay flows from the animal model don't actually link to anything at the moment, I'll make an issue to track this, and then fix this when I'm back working on the soil model.

Fixes #510

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.01%. Comparing base (1b9487d) to head (c8d3ecb).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #562      +/-   ##
===========================================
+ Coverage    94.95%   95.01%   +0.05%     
===========================================
  Files           74       74              
  Lines         4186     4235      +49     
===========================================
+ Hits          3975     4024      +49     
  Misses         211      211              

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[TaranRallings]

Looks good to me! (weeping softly at my desk)